This invention relates to magneto-optic polarization rotator structures which are suitable for inclusion in thin-film integrated optical devices.
When placed between a polarizer and an analyzer, a polarization rotator can serve as an optical isolator, e.g., for isolating a semiconductor radiation source from radiation reflected in an optical communications system. A magneto-optic polarization rotator may include a magnetic garnet medium, e.g., of single-crystal yttrium iron garnet ("YIG", Y.sub.3 Fe.sub.5 O.sub.12), magnetized in the direction of light propagation. For use in guided-wave integrated optics, for example, a rotator medium is preferably formed as a thin film on a substrate.
Background documents for the invention are reviewed as follows:
U.S. Pat. No. 5,031,983, "Apparatus Comprising a Waveguide Magneto-optic Isolator" issued Jul. 16, 1991 to J. F. Dillon et al. discloses an optical waveguide isolator including a magnetic medium in an applied magnetic field. The waveguide is formed as a multi-layer film of modified bismuth yttrium iron garnet (Bi-YIG) on a substrate of gadolinium gallium garnet ("GGG", Gd.sub.3 Ga.sub.5 O.sub.12).
U.S. Pat. No. 4,712,855, "Planar Optical Waveguide and Method of Manufacturing Same", issued Dec. 15, 1987 to W. F. M. Tolksdorf et al. discloses a multi-layer waveguide structure in which the propagation of undesired modes is inhibited. The layers are made of modified yttrium iron garnet materials.
U.S. Pat. No. 3,860,325, "Electric Field Displacing Type Optical Isolator", issued Jan. 14, 1975 to S. Matsushita et al. discloses an optical isolator based on displacement of the light path in a transverse magnetic field. With a suitably placed absorbing medium, light traveling in a waveguide in one direction is absorbed to a greater degree as compared with light traveling in the opposite direction.
U.S. Pat. No. 4,973,119, "Optical Waveguide Isolator" issued Nov. 27, 1990 to K Taki discloses a magneto-optic isolator including a multi-layer mode converter on a substrate. The converter is magnetized perpendicular to the direction of light propagation.
The paper by K. Matsuda et al., "Magnetless Faraday Rotator of (BiY).sub.3 Fe.sub.5 O.sub.12 Waveguide with Strip Magnetic Domains", Applied Physics Letters, Vol. 59, 1991, pp. 507-509 discloses a polarization rotator without a magnet, with striped magnetic domains in a Bi-YIG transmission medium.
The paper by R. Wolfe et al., "Etch-tuned Ridged Waveguide Magneto-optic Isolator", Applied Physics Letters, Vol. 56, 1990, pp. 426-428 discloses chemical etching of a triple-layer Bi-YIG film to reduce the linear birefringence in a polarization rotator, for enhanced blocking of reflected light in a magneto-optic isolator.
The paper by R. Wolfe et al., "Thin-film Garnet Materials with Zero Linear Birefringence for Magneto-optic Waveguide Devices" Journal of Applied Physics, Vol 63, 1988, pp. 3099-3103 describes causes of birefringence in epitaxial films and means for compensation.
The paper by G. A. Prinz, "Hybrid Ferromagnetic Semi-conductor Structures" Science, Vol 250, 1990, pp 1092-1097 discloses the fabrication of single-crystal films of iron and iron alloys on semiconductor substrates.
The paper by M. McGlashan-Powell et al., "Magneto-optic Waveguide Hysteresis Loops of "Planar" Magnetic Garnet Films" Journal of Applied Physics, Vol 66, 1989, pp 3342-3347 discloses a nearly in-plane easy axis of magnetization for Bi-YIG epitaxial layers.
The paper by E. Yablonovitch et al. "Epitaxial Liftoff Technology for OEIC's", American Institute of Physics, American Vacuum Society Series 10, Conference Proceedings, 1991, pp. 154-156 discloses a lift-off technique for transferring an epitaxially grown film from one substrate onto another.
The paper by D. I. Bardos, Journal of Applied Physics, Vol. 40, 1969, pp. 1371-1372 reports on hysteresis studies of iron-cobalt alloys.
Described polarization rotators and magneto-optical isolators use large magnets or solenoids to produce a magnetic field for magnetizing a magneto-optic medium. Such magnetic-field sources are not suited for integrated optics applications.